Corrugated paperboards are made of corrugated sheets bonded through an adhesive that is placed on the flute tips of the medium (fluted sheet) to a panel (flat sheet). In the manufacture of corrugated boards, starch-based adhesives are the most widely used corrugating adhesives. While various optional ingredients may be added to the starch-based adhesive formulations to enhance the adhesive properties, starch-based adhesives typically include several basic raw materials or ingredients; they are water, starch, caustic soda (sodium hydroxide) and borax. Borax is added to the formulation to raise the viscosity and cohesiveness of the starch, and to allow the use of a relatively small amount of the polysaccharide in the formulation (than would otherwise be used in the absence of borax). Borax is typically added in solid powder or granular forms to the aqueous starch-based adhesive mixture during the formation of the adhesive. With the advent of automation, most manufacturing systems use augers to feed the solid borax into the mixer. In these configurations, two problems are generally encountered. Firstly, it is difficult to accurately meter a small amount of solid borax into the aqueous starch mixture. Secondly, the spout located at the extremity of the feeding auger is continuously wet with condensate from the steam released by the hot starch mixture, resulting in the agglomeration of the borax around the spout. This ultimately leads to the clogging of the metering device, thus, the spout requires regular downtime for cleaning.
A solution to the aforementioned issues has been proposed to use organic solvent-based boron-containing liquids instead of solid borax. The boron-containing liquids are organic solvent based solutions containing water-soluble reaction products of boric acid with organic solvent(s) such as amino compounds, e.g., aliphatic polyamines or aminohydroxylated compounds, e.g., alkanolamines, in particular, monoethanolamine. Again, there are two major shortcomings when using organic solvent-based boron-containing solutions. First, these solutions also have fairly high viscosities, thus, accurately metering them into the slurry of the starch mixture during the preparation of the adhesive is also difficult. Secondly, the performance of the solutions used as a boron-containing additive is acceptable only for adhesive bond lines, in the final corrugated board, that do not require water-resistance; this would not meet the need for water resistant corrugated boards, especially when water-resistant starch-based corrugating adhesives have been given significant attentions in recent years.
The basic starch-based corrugating adhesive is not “water-resistant”, that is, the dried glue is readily soluble in water. For example, a regular corrugated board will quickly separate into its paper components if soaked in water for a short period such as from a few minutes to about 3 hours. To impart water resistance to the adhesives, a variety of specialty resins, also known as “water proofing” resins, are typically added to the starch-based adhesives during the preparation thereof. These resins normally include aldehyde functionality, thus, are chemically reactive and tend to increase water resistance of the adhesive bond line by crosslinking with the hydroxyl groups in the starch molecules. However, the concomitant use of a “water proofing” resin and an organic solvent based boron-containing liquid, such as an amine based organic solvent system instead of solid borax, causes the resin to become ineffective and/or to lose its waterproofing attribute due to a competing reaction between aldehyde functionality in the waterproofing resin and amine functionality in the boron-containing liquid: R′—C═O+R—NH2→R′—CH═NR+H2O. As a result, the waterproofing resin is no longer available for crosslinking with the starch to provide water resistant to the adhesive, thereby to the corrugated board.